Human complement receptor type 2 (CR2/CD21) is an ~145 Kd Type I transmembrane protein that serves as a receptor for three classes of ligands. These include C3 cleavage fragments (C3d, C3dg and iC3b), Epstein-Barr virus gp350/220 and CD23. Each of these ligands interacts with the amino-terminal region of the receptor within 2 of 16 repetitive elements that have been designated short consensus repeats (SCRs). SCR-containing proteins that interact with complement C3 and/or C4 are part of a family called the Regulators of Complement Activation (RCA). While the biologic relevance of these proteins is well established, important structure-function characteristics of SCR-containing proteins are poorly understood at the molecular level. We are studying CR2 as a model for receptor-ligand interactions in this family, initially focusing on the amino terminal domain that is designated herein the SCR1-2 domain and which interacts with each of the ligands described above. Preliminary studies using solution as well as x-ray crystallographic techniques have identified several protein-protein interfaces that are likely key to receptor interactions of CR2 with the C3d ligand as well as strongly suggested a model in which the non-ligand-bound free and ligand-bound structures of the SCR1-2 domain differ in their conformation. In addition, recent data have shown that SCRs of CR2 outside of the SCR1-2 domain influence binding with C3d. We now propose to extend these studies by pursuing the following specific aims:Specific Aim #1: Determine the solution phase structure of the CR2 SCR1-2 domain in order to establish the physical relationship between the non-ligand-bound and C3d-ligand-bound receptor states.Specific Aim #2: Establish the relative roles of the three unique protein-protein interfaces apparent in the CR2 SCR1-2:C3d co-crystal structure in C3d ligand binding, signal transduction and the enhancement of in vivo immune responses.Specific Aim #3: Determine the relationship between the C3d, gp350/200 and CD23 ligand binding sites within the CR2 SCR1-2 domain.Specific Aim #4: Establish the solution structure of the CR2 SCR1-15/16 extracellular domain and determine the physical basis for the altered CR2-C3d ligand-receptor binding kinetics when comparing this full length receptor with the CR2 SCR1-2 domain.